There are similarities between television systems conforming to NTSC, PAL M and PAL N television standards which permit the development of a multiple standard TV display based on an existing NTSC receiver design. Cost and production considerations may dictate that a multistandard TV display be engineered for minimal changes to the basic NTSC receiver design.
An existing NTSC receiver design is based on a digital bus controlled integrated circuit which provides most of the required TV receiver sub-systems, for example, IF amplification, video demodulation, chrominance demodulation and decoding, audio demodulation, sync separation and pulse and waveform generation etc. Hence, a multi-standard circuit design must be interfaced to allow control by the existing system, and to provide compatible output signals for reinsertion into the existing receiver video and synchronizing systems.
The bus controlled integrated circuit may include a count down pulse generation system which is locked to the horizontal synchronizing pulses derived from the selected viewing source. The pulse generation system also produces various corrective signal waveforms which may be generated based on horizontal line counts from a vertical reference time. One such signal is a vertical rate parabola which may be use to provide East-West pincushion correction and is controlled and set via the digital bus.
The NTSC, PAL M and PAL N television standards have similar horizontal synchronizing frequencies and also similar color subcarrier frequencies. In addition both NTSC, PAL M have the same field frequency, nominally 59.94 Hz, with the same number of horizontal lines per frame, 525, which yields the same horizontal frequency, nominally 15,734 KHz. However, the
N television standard comprises 625 lines per frame, with a field frequency of 50 Hz. It is these numerical differences which may produce waveform anomalies in a generator based on counting or count down techniques.
In an existing NTSC receiver, pincushion or East-West correction is provided by a vertical rate parabolic shaped signal generated by processing a field rate sawtooth in a squaring circuit. The parabola amplitude is controlled by a feedback loop which is referenced to the instantaneous ramp amplitude occurring at a specific line count. The line count being selected to position the center of the parabola nominally to the center of the field rate ramp or sawtooth. However, when operating in the 50 Hz field rate, PAL N TV standard, the reference line count, or number, is not changed to reflect the greater number of lines in a PAL N field. This results in the generation of a parabolic waveform which is not symmetrically positioned in PAL N field interval. Effectively the parabolic signal is generated to be centered within a 16.66 millisecond field duration, but continues for 20 milliseconds which constitutes the 50 Hz field duration. Thus the peak of the parabola occurs too early in the
N field, and the parabola continues to be generated producing an ending cusp of greater magnitude than that of the start.